1. Field of the Invention
This invention relates in general to processing substrates, and, in particular, to a method and apparatus for simultaneous rapid thermal processing of a plurality of substrates, such as semiconductor wafers.
2. Description of Related Art
Deposition of a film on the surface of a semiconductor substrate is a common step in semiconductor processing. Typically, selected chemical gases are mixed in a deposition chamber containing the semiconductor substrate. Usually, heat is applied to drive the chemical reaction of the gases in the chamber and to heat the surface of the substrate on which the film is deposited.
In deposition processes, it is desirable to maximize substrate throughput, i.e., the number of substrates processed per unit time, while depositing film layers that have uniform properties, e.g., uniform thickness and resistivity for an epitaxial layer. To obtain uniform properties of a deposited film, it is important to maintain the substrate at a uniform temperature during deposition.
A number of different deposition reactors have been developed. Generally, each deposition reactor has a reaction chamber, a substrate handling system, a heat source and temperature control, and a gas delivery system (inlet, exhaust, flow control).
Deposition reactors may be classified according to characteristics of their operation. For instance, a reactor may be either a cold wall or a hot wall reactor. Cold wall reactors are usually preferred because undesirable deposits do not build up on the reaction chamber walls.
A reactor may also be characterized by the amount of time that is required to heat-up, process, and cool-down the substrate. Conventional non-RTP CVD reactors take on the order of 40 to 90 minutes for a complete process cycle of a batch of substrates. An important aspect in obtaining uniform depositions in conventional non-RTP CVD reactors is that the susceptor function as a thermal flywheel. See for example, U.S. Pat. Nos. 4,081,313 and 4,496,609 and the prosecution history for these patents that is publicly available from the U.S. Patent and Trademark Office. A susceptor that functions as a thermal flywheel has a large thermal mass and consequently requires a significant time period to heat that mass to the operating temperature and similarly a significant time to cool that mass to an ambient temperature.
In contrast, rapid thermal process (RTP) reactors typically require only 2 to 15 minutes to process a substrate and consequently cannot utilize the massive susceptors of the conventional CVD reactors. Rapid thermal reactors are characterized by the fact that the process cycle time is significantly less that the process cycle time for a conventional CVD reactor.
Conventional non-RTP CVD reactors have been used to process a plurality of substrates in one batch. See for example, U.S. Pat. No. 5,053,247, entitled "Method for Increasing the Batch Size of a Barrel Epitaxial Reactor and Reactor Produced Thereby," of Gary M. Moore issued on Oct. 1, 1991 and U.S. Pat. No. 5,207,835, entitled "High Capacity Epitaxial Reactor," of Gary M. Moore issued on May 4, 1993, each of which is incorporated herein by reference in its entirety.
RTP reactors have been used to process single substrate batches and also a plurality of substrates in a single batch. See for example, U.S. patent application Ser. No. 08/185,691 entitled "A RAPID THERMAL PROCESSING APPARATUS FOR PROCESSING SEMICONDUCTOR WAFERS," of Gary M. Moore and Katsuhito Nishikawa filed on Jan. 21, 1994, now U.S. Pat. No. 5,683,518, and U.S. patent application Ser. No. 08/007,981 entitled "A RAPID THERMAL PROCESSING APPARATUS FOR PROCESSING SEMICONDUCTOR WAFERS," of Gary M. Moore and Katsuhito Nishikawa filed on Jan. 21, 1993, now U.S. Pat. No. 5,444,217, each of which is incorporated herein by reference in its entirety.
While both substrate size and substrate throughput have increased over the years for rapid thermal processing, the multiple substrate batch sizes in an RTP reactor are less than the batch sizes of a conventional non-RTP CVD barrel reactor for a given substrate diameter. To further reduce the per die cost of RTP, larger RTP batch sizes are required. However, making a larger volume RTP reactor, requires heating of that larger volume to an approximately uniform state prior to initiating processing. This would increase the cycle time not only due to the longer heat-up time, but also due to the increased thermal flywheel effect associated with the larger volumes.
In addition, processing larger batch sizes requires a larger susceptor. However, constructing a large susceptor is problematic due to the inherent thermal flywheel affects of a larger susceptor. Thus, a way is needed to increase the batch size of RTP reactors while maintaining the short cycle times associated with RTP.